Rotary control valves are used in a wide number of process control system applications to control some parameter of a process fluid (this may be a liquid, gas, slurry, etc.). While the process control system may use a control valve to ultimately control the pressure, level, pH or other desired parameter of a fluid, the control valve basically controls the rate of fluid flow.
Typically, a rotary control valves may include a valve body having a fluid inlet and a fluid outlet separated by a ball element which, by rotating about a fixed axis and abutting a seal assembly, controls the amount of fluid flow therethrough. During operation, the process control system, or an operator controlling the control valve manually, rotates the ball element against, or away from a surface of the seal assembly, thereby exposing a flow passage, to provide a desired fluid flow through the inlet and outlet and, therefore, the rotary control valve.
Rotary control valve components, including the ball element and the seal assembly, are typically constructed of metal; this stands especially true when used in high pressure and/or high temperature applications. However, the ball element and seal assembly suffer wear due to the repeated extensive engagement of the ball element and seal assembly during opening and closing of the valve. The problems resulting from the wear include, but are not limited to, diminished life span of the valve components, increased frictional forces between the ball element and the seal assembly, and undesirable leakage between the ball element and the seal assembly. Similarly, because the frictional forces tend to increase as the components become more worn, the dynamic performance and control characteristics within the valve are worsened, resulting in inefficiencies and inaccuracies in the valve.
In the past, attempts have been made to incorporate a biased main seal into the seal assembly to correct the above mentioned problems. Some designs have incorporated a Teflon® radial seal to enhance sealing performance under high-temperature operations. Ball valves having Teflon® radial seals are generally used in operating environments having temperatures up to approximately 550° F. Above 550° F., a graphite piston ring is currently used because Teflon® deteriorates above approximately 550° F. Graphite piston rings, while able to withstand higher temperatures, are do not seal as well as Teflon® radial seals.
Biased main seals, however, form secondary flow paths that require secondary seals to prevent fluid from flowing through the secondary flow path when the valve is closed.
In one example of a known rotary control valve, as illustrated in FIGS. 1-4, a ball valve 20 includes a valve body or housing 30 having a primary flowpath 33 between an inlet 31 and an outlet 32, a seal assembly 50 attached to the housing 30, and a ball element 80 mounted on rotatable shafts 90 and 91 disposed within the housing 30.
The housing 30, having a generally cylindrical shape, defines the primary flowpath 33 for a fluid traveling therethrough. At the bottom of the housing 30, as oriented in FIG. 2, is the outlet 32 of the primary flowpath 33, the outlet 32 being surrounded by an outlet flange 38. In a middle portion of the housing 30, a thru hole 40 penetrates the right wall of the housing 30, and a blind hole 41 opens to the interior of the housing 30, both holes 40 and 41 receive the shafts 90 and 91, respectively. Disposed between the drive shaft 90 and the outer right wall or drive end of the housing 30, is a packing follower 42, a set of packing rings 44, and a bearing 43a. Located on the drive end of housing 30, and engaging with fasteners 35, is an actuator mounting flange 34. Now turning to the top of the housing 30, still as oriented in FIG. 2, is a counterbore 39, creating the inlet 31 of the primary flowpath 33 and, receiving the seal assembly 50. Surrounding the inlet 31 is an inlet flange 36, the inlet flange 36 may be used to fasten or attach the valve 20 to an incoming pipe (not shown).
The seal assembly 50, as shown best in FIG. 4, includes a main seal 64, and a seal housing 52. As mentioned above, the seal assembly 50 is disposed within the counterbore 39 of the housing 30, and more specifically, an exterior surface 54 of the seal housing 52 is fixedly attached within the counterbore 39. On an interior surface 53 of the seal housing 52, is a pair of annular shoulders 55a & 55b, which receive a dynamic C-seal 60 and a resilient biasing member, such as a wave spring 70, respectively. The C-seal 60 and the resilient member 70 connect the main seal 64 to the seal housing 52. The resilient member 70 biases the main seal 64 toward the ball element 80, by the addition of which a secondary flowpath 77 between the main seal 64 and the seal housing 52 is created. The C-seal creates a flow restriction of the fluid through the secondary flowpath 77. The C-seal is trapped between an annular shoulder 74 on the main seal and an annular shelf 76 on the seal housing 52. An opening of the C-seal 63 faces away from the ball 80 and toward the incoming fluid.
Abutting the main seal 64, when the valve 20 is in the closed position, is the ball element 80 (FIG. 4). The ball element 80 includes a spherical surface 82 that engages the main seal 64 when the valve is in the closed position. Attached to the ball element 80, through thru holes 84a & 84b are the follower shaft 91 and the drive shaft 90, respectively.
To close the valve, the ball element 80 is rotated to abut the main seal 64, thereby creating a flow restriction of the primary flowpath 33 at a contact point 66. As shown in FIG. 4, when the ball element 80 presses against the main seal 64, the main seal 64 may be displaced into the seal housing 52 by compressing the resilient member 70. To ensure proper movement and operation of the main seal 64, relative to the ball element 80 and the seal housing 52, a predetermined or calculated gap 71 created between the main seal 64 and the seal housing 52. The gap 71 is set to ensure that the main seal 64 contacts the ball element 80, when the valve 20 is in the closed position. Working in combination with the gap 71 to ensure proper movement and operation of the valve 20, is a gap 73 created between the main seal 64 and the housing 30. The gap 73 ensures that the main seal 64 comes into direct contact with the housing 30, at the proper time, when the valve 20 is opening and closing.
As the ball element 80 rotates toward the closed position, the ball element 80 contacts the main seal 64, thereby causing the gap 71 to become smaller as the ball element 80 rotates further into the fully closed position. Also shown in FIG. 4 is the secondary flowpath 77, created between the main seal 64 and the seal housing 52 for accommodation of the resilient member 70.
When the ball valve 20 is in the closed position, high pressure forces are created at the inlet 31. The increase of pressure may force the process fluid to bypass the primary flowpath restriction and be forced through the secondary flowpath 77. Preventing the fluid from penetrating through the secondary flowpath 77 is the dynamic C-seal 60. The main seal 64 will continue to be biased against the ball element 80, until the main seal 64 is stopped, or the resilient member 70 is fully decompressed.
One problem with known rotary control valves, such as the ball valve illustrated in FIGS. 1-4, is that the secondary flowpath, created by the biased main seal, must itself be sealed when the valve is in a closed position. This additional seal, often in the form of a dynamic radial seal, adds complexity, difficulty in manufacturing, and additional points of failure to known ball valves.